The present invention relates generally to methods and materials for treating gram-positive bacterial infections by administration of bactericidal/permeability-increasing (BPI) protein products alone, or in combination with antibiotic substances. Also disclosed are analogous methods and materials involving use of lipopolysaccharide binding protein (LBP) derivatives.
BPI is a protein isolated from the granules of mammalian polymorphonuclear leukocytes (PMNs or neutrophils), which are blood cells essential in the defense against invading microorganisms. Human BPI protein has been isolated from PMNs by acid extraction combined with either ion exchange chromatography [Elsbach, J. Biol. Chem., 254: 11000 (1979)] or E. coli affinity chromatography [Weiss, et al., Blood, 69: 652 (1987)]. BPI obtained in such a manner is referred to herein as natural BPI and has been shown to have potent bactericidal activity against a broad spectrum of gram-negative bacteria. The molecular weight of human BPI is approximately 55,000 daltons (55 kD). The amino acid sequence of the entire human BPI protein and the nucleic acid sequence of DNA encoding the protein have been reported in FIG. 1 of Gray et at., J. Biol. Chem., 264: 9505 (1989), incorporated herein by reference. The Gray et al. amino acid sequence is set out in SEQ ID NO: 69 hereto.
BPI is a strongly cationic protein. The N-terminal half of BPI accounts for the high net positive charge; the C-terminal half of the molecule has a net charge of -3. [Elsbach and Weiss (1981), supra.] A proteolytic N-terminal fragment of BPI having a molecular weight of about 25 kD has an amphipathic character, containing alternating hydrophobic and hydrophilic regions. This N-terminal fragment of human BPI possesses the anti-bacterial efficacy of the naturally-derived 55 kD human BPI holoprotein. [Ooi et al., J. Bio. Chem., 262: 14891-14894 (1987)]. In contrast to the N-terminal portion, the C-terminal region of the isolated human BPI protein displays only slightly detectable anti-bacterial activity against gram-negative organisms. [Ooi et al., J. Exp. Med., 174: 649 (1991).] An N-terminal BPI fragment of approximately 23 kD, referred to as "rBPI.sub.23," has been produced by recombinant means and also retains anti-bacterial activity against gram-negative organisms. Gazzano-Santoro et al., Infect. Immun. 60: 4754-4761 (1992).
The bactericidal effect of BPI has been reported to be highly specific to gram-negative species, e.g., in Elsbach and Weiss, Inflammation: Basic Principles and Clinical Correlates, eds. Gallin et al., Chapter 30, Raven Press, Ltd. (1992). BPI is commonly thought to be non-toxic for other microorganisms, including yeast, and for higher eukaryotic cells. Elsbach and Weiss (1992), supra, reported that BPI exhibits anti-bacterial activity towards a broad range of gram-negative bacteria at concentrations as low as 10.sup.-8 to 10.sup.-9 M, but that 100- to 1,000-fold higher concentrations of BPI were non-toxic to all of the gram-positive bacterial species, yeasts, and higher eukaryotic cells tested at that time. It was also reported that BPI at a concentration of 10.sup.-6 M or 160 .mu.g/ml had no toxic effect, when tested at a pH of either 7.0 or 5.5, on the gram-positive organisms Staphylococcus aureus (four strains), Staphylococcus epidermidis, Streptococcus faecalis, Bacillus subtilis, Micrococcus lysodeikticus, and Listeria monocytogenes. BPI at 10.sup.-6 M reportedly had no toxic effect on the fungi Candida albicans and Candida parapsilosis at pH 7.0 or 5.5, and was non-toxic to higher eukaryotic cells such as human, rabbit and sheep red blood cells and several human tumor cell lines. See also Elsbach and Weiss, Advances in Inflammation Research, ed. G. Weissmann, Vol. 2, pages 95-113 Raven Press (1981). This reported target cell specificity was believed to be the result of the strong attraction of BPI for lipopolysaccharide (LPS), which is unique to the outer membrane (or envelope) of gram-negative organisms.
The precise mechanism by which BPI kills gram-negative bacteria is not yet completely elucidated, but it is believed that BPI must first bind to the surface of the bacteria through electrostatic and hydrophobic interactions between the cationic BPI protein and negatively charged sites on LPS. LPS has been referred to as "endotoxin" because of the potent inflammatory response that it stimulates, i.e., the release of mediators by host inflammatory cells which may ultimately result in irreversible endotoxic shock. BPI binds to lipid A, reported to be the most toxic and most biologically active component of LPS.
In susceptible gram-negative bacteria, BPI binding is thought to disrupt LPS structure, leading to activation of bacterial enzymes that degrade phospholipids and peptidoglycans, altering the permeability of the cell's outer membrane, and initiating events that ultimately lead to cell death. [Elsbach and Weiss (1992), supra]. BPI is thought to act in two stages. The first is a sublethal stage that is characterized by immediate growth arrest, permeabilization of the outer membrane and selective activation of bacterial enzymes that hydrolyze phospholipids and peptidoglycans. Bacteria at this stage can be rescued by growth in serum albumin supplemented media [Mannion et al., J. Clin. Invest., 85: 853-860 (1990)]. The second stage, defined by growth inhibition that cannot be reversed by serum albumin, occurs after prolonged exposure of the bacteria to BPI and is characterized by extensive physiologic and structural changes, including apparent damage to the inner cytoplasmic membrane.
Initial binding of BPI to LPS leads to organizational changes that probably result from binding to the anionic groups in the KDO region of LPS, which normally stabilize the outer membrane through binding of Mg.sup.++ and Ca.sup.++. Attachment of BPI to the outer membrane of gram-negative bacteria produces rapid permeabilization of the outer membrane to hydrophobic agents such as actinomycin D. Binding of BPI and subsequent gram-negative bacterial killing depends, at least in part, upon the LPS polysaccharide chain length, with long O-chain bearing, "smooth" organisms being more resistant to BPI bactericidal effects than short O-chain bearing, "rough" organisms [Weiss et al., J. Clin. Invest. 65: 619-628 (1980)]. This first stage of BPI action, permeabilization of the gram-negative outer envelope, is reversible upon dissociation of the BPI, a process requiring the presence of divalent cations and synthesis of new LPS [Weiss et al., J. Immunol. 132: 3109-3115 (1984)]. Loss of gram-negative bacterial viability, however, is not reversed by processes which restore the envelope integrity, suggesting that the bactericidal action is mediated by additional lesions induced in the target organism and which may be situated at the cytoplasmic membrane (Mannion et al., J. Clin. Invest. 86: 631-641 (1990)). Specific investigation of this possibility has shown that on a molar basis BPI is at least as inhibitory of cytoplasmic membrane vesicle function as polymyxin B (In't Veld et al., Infection and Immunity 56: 1203-1208 (1988)) but the exact mechanism as well as the relevance of such vesicles to studies of intact organisms has not yet been elucidated.
BPI is also capable of neutralizing the endotoxic properties of LPS to which it binds. Because of its bactericidal properties for gram-negative organisms and its ability to neutralize LPS, BPI can be utilized for the treatment of mammals suffering from diseases caused by gram-negative bacteria, such as bacteremia or sepsis.
U.S. Pat. No. 5,198,541 discloses recombinant genes encoding and methods for expression of BPI proteins, including BPI holoprotein and fragments of BPI. It also describes the use of N-terminal fragments of BPI protein for co-treatment of gram-negative bacterial diseases with certain antibiotics, specifically penicillin, cephalosporins, rifampicin and actinomycin D.
Antibiotics are natural chemical substances of relatively low molecular weight produced by various species of microorganisms, such as bacteria (including Bacillus species), actinomycetes (including Streptomyces) and fungi, that inhibit growth of or destroy other microorganisms. Substances of similar structure and mode of action may be synthesized chemically, or natural compounds may be modified to produce semi-synthetic antibiotics. These biosynthetic and semi-synthetic derivatives are also effective as antibiotics. The major classes of antibiotics are (1) the .beta.-lactams, including the penicillins, cephalosporins and monobactams; (2) the aminoglycosides, e.g., gentamicin, tobramycin, netilmycin, and amikacin; (3) the tetracyclines; (4) the sulfonamides and trimethoprim; (5) the fluoroquinolones, e.g., ciprofloxacin, norfloxacin, and ofloxacin; (6) vancomycin; (7) the macrolides, which include for example, erythromycin, azithromycin, and clarithromycin; and (8)other antibiotics, e.g., the polymycins, chloramphenicol and the lincosamides.
Antibiotics accomplish their anti-bacterial effect through several mechanisms of action which can be generally grouped as follows: (1) agents acting on the bacterial cell wall such as bacitracin, the cephalosporins, cycloserine, fosfomycin, the penicillins, ristocetin, and vancomycin; (2) agents affecting the cell membrane or exerting a detergent effect, such as colistin, novobiocin and polymyxins; (3) agents affecting cellular mechanisms of replication, information transfer, and protein synthesis by their effects on ribosomes, e.g., the aminoglycosides, the tetracyclines, chloramphenicol, clindamycin, cycloheximide, fucidin, lincomycin, puromycin, rifampicin, other streptomycins, and the macrolide antibiotics such as erythromycin and oleandomycin; (4) agents affecting nucleic acid metabolism, e.g., the fluoroquinolones, actinomycin, ethambutol, 5-fluorocytosine, griseofulvin, rifamycins; and (5) drugs affecting intermediary metabolism, such as the sulfonamides, trimethoprim, and the tuberculostatic agents isoniazid and para-aminosalicylic acid. Some agents may have more than one primary mechanism of action, especially at high concentrations. In addition, secondary changes in the structure or metabolism of the bacterial cell often occur after the primary effect of the antimicrobial drug.
The penicillins have a characteristic double-ring system composed of a .beta.-lactam ring, which provides the antibacterial activity, and a thiazolidene ring. The penicillins are differentiated by a single side chain that is unique for each penicillin. The compounds are bactericidal and act by inhibiting bacterial transpeptidase, an enzyme involved in synthesis of the bacterial cell wall. Because of their mechanism of action, penicillins are generally active against growing, but not resting, cells. Penicillins, especially penicillin G, have largely gram-positive activity; the relative insensitivity of gram-negative rods to penicillin G and several other penicillins is probably due to the permeability barrier of the outer membrane of gram-negative bacteria. Ampicillin, carbenicillin, ticarcillin, and some other penicillins are active against gram-negative bacteria because they can pass through this outer membrane. Penicillins have relatively few adverse effects, the most important of which are the hypersensitivity (allergic) reactions. These compounds are widely distributed in the body, but do not enter cells and do not usually accumulate in CSF.
Bacterial resistance to the penicillins is by production of the enzyme .beta.-lactamase, which catalyzes hydrolysis of the .beta.-lactam ring. The percentage of bacteria resistant to penicillin has risen to about 80%. Several penicillins, including methicillin, oxacillin, cloxacillin, dicloxacillin and nafcillin, are not affected by the .beta.-lactamase of staphylococci. These antibiotics are useful against most .beta.-lactamase-producing species of Staphylococcus. However, a small number of species are resistant even to these penicillins. Some penicillins, amoxicillin and ticarcillin, are marketed in combination with clavulanic acid, which is a .beta.-lactamase inhibitor that covalently binds to the enzyme and prevents it from hydrolyzing the antibiotics. Another inhibitor, sulbactam, is marketed in combination with ampicillin.
The cephalosporins are characterized by a .beta.-lactam ring, like the penicillins, but have an adjacent dihydrothiazine ring instead of a thiazolidene ring. For convenience, these compounds are generally classified by generations. The first generation includes cephalothin, cephapirin, cefazolin, cephalexin, cephradine and cefadroxil. These drugs generally have excellent gram-positive activity except for enterococci and methicillin-resistant staphylococci, and have only modest gram-negative coverage. The second generation includes cefamandole, cefoxitin, ceforanide, cefuroxime, cefuroxime axetil, cefaclor, cefonicid and cefotetan. This generation generally loses some gram-positive activity by weight and gains limited gram-negative coverage. The third generation includes cefotaxime, moxalactam, ceftizoxime, ceftriaxone, cefoperazone and ceftazidime. These compounds generally sacrifice further gram-positive activity by weight but gain substantial gram-negative coverage against Enterobacter and sometimes are active against Pseudoraonas. The cephalosporins bind to penicillin-binding proteins with varying affinity. Once binding occurs, protein synthesis is inhibited. Cephalosporins are usually well tolerated; adverse effects include hypersensitivity reactions and gastrointestinal effects. Cephalosporins may interact with nephrotoxic drugs, particularly aminoglycosides, to increase toxicity. Resistance to cephalosporins is mediated by several mechanisms, including production of .beta.-lactamase, although some strains that do not produce .beta.-lactamase are nevertheless resistant.
Imipenem is a N-formimidoyl derivative of the mold product thienamycin. It contains a .beta.-lactam ring and somewhat resembles penicillin except for differences in the second ring. It has activity against both gram-positive and gram-negative organisms and is resistant to most .beta.-lactamases, although not those from Pseudomonas. It is marketed in combination with cilastin, a compound that inhibits inactivation of imipenem in the kidney by renal dihydropeptidase I enzyme. Cilastin increases the concentration of imipenem in urine, although not in blood.
Aztreonam is the first of a new group of antibiotics referred to as the monobactams. These agents have a .beta.-lactam ring but lack the second ring characteristic of the penicillins and cephalosporins. It acts by binding to penicillin-binding proteins, and produces long, filamentous bacterial shapes that eventually lyse. Aztreonam is active only against aerobic gram-negative bacteria, is susceptible to inactivation by some .beta.-lactamases, and has few adverse effects.
The aminoglycosides contain amino sugars linked to an aminocyclitol ring by glycosidic bonds. They have similar mechanisms of action and properties, but differ somewhat in spectrum of action, toxicity, and susceptibility to bacterial resistance. The compounds are bactericidal, with activity against both gram-positive and gram-negative organisms, and act by binding to proteins on the 30S ribosome of bacteria and inhibiting protein synthesis. The aminoglycosides also bind to isolated LPS and have a very weak outer membrane permeabilizing effect. [Taber et at., Microbiological Reviews 53: 439-457 (1987)); Kadurugamuwa et al., Antmicrobial Agents and Chemotherapy, 37: 715-721 (1993); Vaara, Microbiological Reviews 56: 395-411 (1992)]. This class of antibiotics includes amikacin, gentamicin, kanamycin, neomycin, netilmycin, paromomycin and tobramycin. The aminoglycosides are usually reserved for more serious infections because of severe adverse effects including ototoxicity and nephrotoxicity. There is a narrow therapeutic window between the concentration required to produce a therapeutic effect, e.g., 8 .mu.g/ml for gentamicin, and the concentration that produces a toxic effect, e.g., 12 .mu.g/ml for gentamicin. Neomycin in particular is highly toxic and is never administered parenterally.
Tetracyclines have a common four-ring structure and are closely congeneric derivatives of the polycyclic naphthacenecarboxamide. The compounds are bacteriostatic, and inhibit protein synthesis by binding to the 30S subunit of microbial ribosomes and interfering with attachment of aminoacyl tRNA. The compounds have some activity against both gram-positive and gram-negative bacteria; however, their use is limited because many species are now relatively resistant. Adverse effects include gastrointestinal effects, hepatotoxicity with large doses, and nephrotoxicity in some patients. This antibiotic class includes tetracycline, chlortetracycline, demeclocycline, doxycycline, methacycline, minocycline and oxytetracycline.
The sulfonamides are derivatives of sulfanilamide, a compound similar in structure to para-aminobenzoic acid (PABA), which is an essential precursor for bacterial synthesis of folic acid. The compounds are generally bacteriostatic, and act by competitively inhibiting incorporation of PABA into tetrahydrofolic acid, which is a required cofactor in the synthesis of thymidines, purines and DNA. Sulfonamides have a wide range of activity against gram-positive and gram-negative bacteria, but their usefulness has diminished with increasingly high prevalence of bacterial resistance. The sulfonamide class of antibiotics includes sulfacytine, sulfadiazine, sulfamethizole, sulfisoxazole, sulfamethoxazole, sulfabenzamide and sulfacetamide. Adverse effects include hypersensitivity reactions and occasional hematological toxicity.
Trimethoprim is an inhibitor of the dihydrofolate reductase enzyme, which converts dihydrofolic to tetrahydrofolic acid, a required factor for DNA synthesis. Adverse effects include gastrointestinal distress and rare hematological toxicity. Trimethoprim is also available in combination with sulfamethoxazole (also known as co-trimoxazole). The combination is usually bactericidal, although each agent singly is usually bacteriostatic. The combination is the drug of choice for Salmonella infections, some Shigella infections, E. coli traveler's diarrhea and Pneumocystis carinii pneumonia.
The fluoroquinolones and quinolones are derivatives of nalidixic acid, a naphthyridine derivative. These compounds are bactericidal, and impair DNA replication, transcription and repair by binding to the DNA and interfering with DNA gyrase, an enzyme which catalyzes negative supercoiling of DNA. The fluoroquinolones, which include norfloxacin, ciprofloxacin, and ofloxacin, and the quinolones, which include cinoxacin, have a broad spectrum of antimicrobial activity against gram-negative and gram-positive organisms. These compounds distribute widely through extravascular tissue sites, have a long serum half-life, and present few adverse effects. Because of their effect on DNA, the drugs are contraindicated in pregnant patients and in children whose skeletal growth is incomplete.
Vancomycin is a glycopeptide, with a molecular weight of about 1500, produced by a fungus. It is primarily active against gram-positive bacteria. The drug inhibits one of the final steps in synthesis of the bacterial cell wall, and is thus effective only against growing organisms. It is used to treat serious infections due to gram-positive cocci when penicillin G is not useful because of bacterial resistance or patient allergies. Vancomycin has two major adverse effects, ototoxicity and nephrotoxicity. These toxicities can be potentiated by concurrent administration of another drug with the same adverse effect, such as an aminoglycoside.
The macrolides are bacteriostatic and act by binding to the 50S subunit of 70S ribosomes, resulting in inhibition of protein synthesis. They have a broad spectrum of activity against gram-positive and gram-negative bacteria and may be bacteriostatic or bactericidal, depending on the concentration achieved at sites of infection. The compounds distribute widely in body fluids. Adverse effects include gastrointestinal distress and rare hypersensitivity reactions. The most common macrolide used is erythromycin, but the class includes other compounds such as clarithromycin and azithromycin.
The polymyxins are a group of closely related antibiotic substances produced by strains of Bacillus polymyxa. These drugs, which are cationic detergents, are relatively simple, basic peptides with molecular weights of about 1000. Their antimicrobial activity is restricted to gram-negative bacteria. They interact strongly with phospholipids and act by penetrating into and disrupting the structure of cell membranes. Polymyxin B also binds to the lipid A portion of endotoxin and neutralizes the toxic effects of this molecule. Polymyxin B has severe adverse effects, including nephrotoxicity and neurotoxicity, and should not be administered concurrently with other nephrotoxic or neurotoxic drugs. The drug thus has limited use as a therapeutic agent because of high systemic toxicity, but may be used for severe infections, such as Pseudomonas aeruginosa meningitis, that respond poorly to other antibiotics.
Chioramphenicol inhibits protein synthesis by binding to the 50S ribosomal subunit and preventing binding of aminoacyl tRNA. It has a fairly wide spectrum of antimicrobial activity, but is only reserved for serious infections, such as meningitis, typhus, typhoid fever, and Rocky Mountain spotted fever, because of its severe and fatal adverse hematological effects. It is primarily bacteriostatic, although it may be bactericidal to certain species.
Lincomycin and clindamycin are lincosamide antimicrobials. They consist of an amino acid linked to an amino sugar. Both inhibit protein synthesis by binding to the 50S ribosomal subunit. They compete with erythromycin and chloramphenicol for the same binding site but in an overlapping fashion. They may be bacteriostatic or bactericidal, depending on relative concentration and susceptibility. Gastrointestinal distress is the most common side effect. Other adverse reactions include cutaneous hypersensitivity, transient hematological abnormalities, and minor elevations of hepatic enzymes. Clindamycin is often the drug of choice for infections caused by anaerobic bacteria or mixed aerobic/anaerobic infections, and can also be used for susceptible aerobic gram-positive cocci.
Some drugs, e.g. aminoglycosides, have a small therapeutic window. For example, 2 to 4 .mu.g/ml of gentamicin or tobramycin may be required for inhibition of bacterial growth, but peak concentrations in plasma above 6 to 10 .mu.g/ml may result in ototoxicity or nephrotoxicity. These agents are more difficult to administer because the ratio of toxic to therapeutic concentrations is very low. Antimicrobial agents that have toxic effects on the kidneys and that are also eliminated primarily by the kidneys, such as the aminoglycosides or vancomycin, require particular caution because reduced elimination can lead to increased plasma concentrations, which in turn may cause increased toxicity. Doses of antimicrobial agents that are eliminated by the kidneys must be reduced in patients with impaired renal function. Similarly, dosages of drugs that are metabolized or excreted by the liver, such as erythromycin, chloramphenicol, or clindamycin, must be reduced in patients with decreased hepatic function.
Antibiotic resistance in bacteria is an increasingly troublesome problem. The accelerating development of antibiotic-resistant bacteria, intensified by the widespread use of antibiotics in farm animals and overprescription of antibiotics by physicians, has been accompanied by declining research into new antibiotics with different modes of action. [Science, 264: 360-374 (1994).]Antibiotic resistance, once acquired, can be rapidly spread to other bacteria, including bacteria of a different species. There are some species of bacteria that are resistant to all but one antibiotic; it may be only a matter of time before the appearance of bacterial strains that are resistant to all antibiotics.
Bacteria acquire resistance to antibiotics through several mechanisms: (1) production of enzymes that destroy or inactivate the antibiotic [Davies, Science, 264: 375-381 (1994)]; (2) synthesis of new or altered target sites on or within the cell that are not recognized by the antibiotic [Spratt, Science, 264: 388-393 (1994)]; (3) low permeability to antibiotics, which can be reduced even further by altering cell wall proteins, thus restricting access of antibiotics to the bacterial cytoplasmic machinery; (4) reduced intracellular transport of the drug; and (5) increased removal of antibiotics from the cell via membrane-associated pumps [Nikaido, Science, 264: 382-387 (1994)].
The susceptibility of a bacterial species to an antibiotic is generally determined by two microbiological methods. A rapid but crude procedure uses commercially available filter paper disks that have been impregnated with a specific quantity of the antibiotic drug. These disks are placed on the surface of agar plates that have been streaked with a culture of the organism being tested, and the plates are observed for zones of growth inhibition. A more accurate technique, the broth dilution susceptibility test, involves preparing test tubes containing serial dilutions of the drug in liquid culture media, then inoculating the organism being tested into the tubes. The lowest concentration of drug that inhibits growth of the bacteria after a suitable period of incubation is reported as the minimum inhibitory concentration.
The resistance or susceptibility of an organism to an antibiotic is determined on the basis of clinical outcome, i.e., whether administration of that antibiotic to a subject infected by that organism will successfully cure the subject. While an organism may literally be susceptible to a high concentration of an antibiotic in vitro, the organism may in fact be resistant to that antibiotic at physiologically realistic concentrations. If the concentration of drug required to inhibit growth of or kill the organism is greater than the concentration that can safely be achieved without toxicity to the subject, the microorganism is considered to be resistant to the antibiotic. To facilitate the identification of antibiotic resistance or susceptibility using in vitro test results, the National Committee for Clinical Laboratory Standards (NCCLS) has formulated standards for antibiotic susceptibility that correlate clinical outcome to in vitro determinations of the minimum inhibitory concentration of antibiotic.
There continues to exist a need in the art for new antimicrobial, and especially anti-gram-positive microbial, methods and materials. Products and methods responsive to this need would ideally involve substantially non-toxic compounds available in large quantities by means of synthetic or recombinant methods. Ideal compounds would have bactericidal or bacteriostatic activity when administered or applied as the sole anti-microbial agent. Such compounds ideally would also be useful in combinative therapies with other anti-microbial agents, particularly where potentiating effects are provided.